Oxygen Vacancy-Induced Room Temperature Ferromagnetism and

Article pubs.acs.org/JPCC

Oxygen Vacancy-Induced Room Temperature Ferromagnetism and Magnetoresistance in Fe-Doped In2O3 Films Yukai An,*,† Yuan Ren,† Dongyan Yang,† Zhonghua Wu,‡ and Jiwen Liu*,† †

Tianjin Key Laboratory for Photoelectric Materials and Devices; School of Material Science and Engineering, Tianjin University of Technology, Tianjin 300384, China ‡ Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: The (In1−xFex)2O3 films (0 ≤ x ≤ 0.09) were prepared by RF-magnetron sputtering. Structural, magnetic and transport properties of the films were investigated systematically both experimentally and theoretically. X-ray absorption spectroscopy (XAS) and multiple-scattering ab initio theoretical calculations reveal that Fe dopant atoms are substitutionally incorporated into In2O3 lattice with a mixed-valence (Fe2+/Fe3+) and form FeIn1+2 VO complex. All the films display room temperature ferromagnetism and the saturated magnetization (Ms) increases monotonically with the increase of Fe concentration. The Mott variable range hopping (VRH) transport behavior dominates the conduction mechanism of the films at low temperature, confirming that the carriers are localized. The Fe doping has profound effects on the positive and negative MR contributions. The positive MR contribution becomes more pronounced with Fe doping, reflecting the occurrence of spin polarization and stronger s−d exchange interaction. The bound magnetic polarons (BMPs) associated with oxygen vacancy can be considered to play an important role in achieving the ferromagnetic order of the (In1−xFex)2O3 films. The variation of Ms with Fe doping has a strong correlation with the localization radius ξ of carriers and the characteristic hopping temperature T0, indicating that the change of localization effect can remarkably influence the ferromagnetic order of the (In1−xFex)2O3 films.

1. INTRODUCTION Recently, dilute magnetic semiconductors (DMSs) have provoked considerable interest because of their technological applications in spintronics.1,2 Since Dietl et al.3 predicted that room-temperature ferromagnetic order can be obtained in ZnO-based DMSs, transition metal (TM) doped ZnO,4,5 TiO2,6,7 SnO2,8,9 and In2O310−12 have been reported to exhibit room temperature ferromagnetism. Among these hosts, In2O3 has triggered much attention due to good electrical conductivity and high optical transparency. However, ferromagnetism in In2O3-based DMSs often results from magnetic metal clusters or secondary magnetic phases and is very sensitive to the growth conditions, such as growth temperature, oxygen pressure, and TM doping concentration. So many contradictory results are reported in different research teams. Xu et al. prepared Fe and Sn codoped In2O3 powders, and found that the observed ferromagnetism is partially due to the precipated Fe3O4 nanoparticles.13 However, Peleckis et al. found that Fedoped In2O3 bulk samples exhibited paramagnetic behavior at room temperature.14 Yoo et al. reported room temperature ferromagnetic Fe and Cu codoped In2O3 bulk sample and attributed the observed ferromagnetism to the multivalence of Fe ions.15 While Chu et al. failed to found the room temperature ferromagnetism in the Fe and Cu codoped In2O3 prepared by coprecipitation synthesis method.16 To understand the exact mechanism of observed ferromagnetism, superexchange, double exchange, RKKY exchange and bound magnetic polaron (BMP) models et al. have been proposed. © XXXX American Chemical Society

Recently, numerous experimental results also suggested that defects in In2O3-based DMSs may play a crucial role in achieving room temperature ferromagnetic order.17,18 Despite the results accumulated on the electronic structure and magnetic interaction, the microscopic origin and underlying mechanism for ferromagnetism in In2O3-based DMSs still remain controversial. For examples, Yan et al. prepared polycrystalline Fe-doped In2O3 powders, and found that the observed room temperature ferromagnetism is related to the oxygen vacancy-induced Fe2+ ions through the hybridization between Fe 4s and Fe 3d states.19 Xing et al. reported room temperature ferromagnetic Fe-doped In2O3 films and used a modified F-center model to explain the oxygen vacancy mediated intrinsic ferromagnetism.20 Li et al. prepared the bulk polycrystalline Fe and Sn codoped In2O3 samples, and concluded that room temperature ferromagnetism originates from oxygen vacancies.21 However, Panguluri et al. deposited the Cr-doped In2O3 films, and suggested that the observed room temperature ferromagnetic order is carrier mediated.22 In this paper, we systematically investigated the local Fe structure, magnetic and transport properties of the (In1−xFex)2O3 films grown by magnetron sputtering. The XAS techniques combined with multiple-scattering ab initio theoretical calculations were used to determine the local Fe Received: December 31, 2014 Revised: January 31, 2015

A

DOI: 10.1021/jp513016q J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

smaller ionic radii of Fe2+ and Fe3+ ions (0.92 and 0.79 Å, respectively) compared to that of In3+ ions (0.94 Å).23 Figure. 2a shows the Fe K-edge XANES spectra for the (In1−xFex)2O3 film as well as standard Fe metal and Fe oxides (FeO, Fe2O3, and Fe3O4) foils. The experimental XANES spectra for the (In1−xFex)2O3 films with x = 0.03, 0.07, and 0.09 have very little difference, therefore, only one (x = 0.09) was shown. The spectral differences between the experimental one and standard samples are substantial, suggesting that the existence of Fe metal and Fe oxides in the film can be safely excluded. It is clear that the position of absorption edge of the film lies between that of FeO and of Fe2O3 and has close proximity to that of Fe3O4, indicating that there exists a mixedvalent state (Fe2+/Fe3+) of Fe ions. Parts b and c of Figure 2 show the Fe K-edge EXAFS k3χ(k) oscillation functions and their Fourier transform (FT) curves for the (In1−xFex)2O3 film with x = 0.09 as well as Fe, FeO, Fe2O3, and Fe3O4 foils. It is obvious that the k3χ(k) oscillation shapes of Fe metal, FeO, Fe3O4 and Fe2O3 are obviously different from that of the (In1−xFex)2O3 film with x = 0.09, indicating a different local structure around doped Fe atoms. The FT curve of the (In1−xFex)2O3 film with x = 0.09 presents three strong peaks at around 1.53, 2.66, and 3.19 Å, which are corresponding to the first (Fe−O), the second (Fe−In) and the third (Fe−In) coordination shell of Fe. The positions of these peaks are completely different from Fe metal (2.24 and 3.63 Å), FeO (1.59 and 2.62 Å), Fe2O3 (1.40 and 2.58 Å), and Fe3O4 (1.40, 2.57, and 3.08 Å). These again reveal that the doped Fe atoms are incorporated into In2O3 lattice without forming Fe metal and Fe oxide secondary phases. To obtain structural parameters around Fe atoms in the (In1−xFex)2O3 film with x = 0.09. We quantitatively fitted the first (Fe−O), the second (Fe−In), and the third (Fe−In) coordination shell of Fe in the range from 0.4 to 4.0 Å, as shown in the solid line of Figure 2c. The best fit to EXAFS data clearly shows that the doped Fe atoms are incorporated into the In1 sites of the In2O3 lattice. The fitted structural parameters are shown in Table 1. One can see that the substitution of Fe results in a large contraction of the firstshell around the Fe (RFe−O = 2.03 Å), compared to that in pure In2O3 (RIn−O = 2.19 Å). The shortening of the Fe−O bond length and a large Debye−Waller (DW) factor σFe−O2 in the first coordination shell around Fe dopant are related to the relaxation of oxygen environment of Fe ions upon substitution. However, this relaxation does not largely distort the second Fe−In and the third Fe−In coordination shell and cause the obvious increase in the corresponding DW factor σ2. The XANES spectral features are highly sensitive to structural defects, such as oxygen vacancies and In interstitials (Ini) around doped TM ions in In2O3-based DMSs. The simulations of Fe K-edge XANES spectra were calculated by the real-space multiple-scattering approach using FEFF 9.0 code with a cluster size of 120 atoms. Parts a and b of Figure 3 show the experimental XANES spectrum for the (In1−xFex)2O3 film with x = 0.09 as well as the calculated XANES spectra for six representative models: model M1, FeIn1 plus In interstitial (FeIn1+Ini); model M2, FeIn2 plus In interstitial (FeIn2 + Ini); model M3, FeIn2 plus O vacancy (FeIn2 + VO); model M4, FeIn1 plus two O vacancy (FeIn1 + 2VO); model M5, FeIn1 plus O vacancy (FeIn1 + VO); model M6, substitutional Fe (FeIn1). Evidently, the calculated XANES spectra of M1, M2, and M3 show quite different features from the experimental spectrum. However, the calculated XANES spectra of M4, M5, and M6 show features similar to the experimental one. The calculated

structure as well as the existence of structural defects around the doped Fe atoms. The objective is to establish the correlations among the structural defect, ferromagnetism and transport properties in the (In1−xFex)2O3 films. These results can give new sights for understanding the mechanism of magnetic interactions, and reveal that the donor oxygen vacancies (Vo) are the key in inducing room temperature ferromagnetism of In2O3-based DMSs.

2. EXPERIMENTAL DETAILS The (In1−xFex)2O3 (0 ≤ x ≤ 0.09) films with a thickness of 340 nm were grown on Si(001) substrates using RF-magnetron sputtering with a base pressure of 7 × 10−5 Pa. The films were sputtered at an Ar pressure of 0.8 Pa during deposition. The In2O3 target was synthesized using the sol−gel method and calcinations treatment. The obtained In2O3 target containing some small Fe pieces was used to prepare the (In1−xFex)2O3 films. By changing the number of small Fe pieces, the (In1−xFex)2O3 films with x = 0, 0.03, 0.07, and 0.09 were readily prepared. The Fe concentration in the (In1−xFex)2O3 films was estimated using the energy dispersive spectroscopy (EDS). The crystalline structures of films were characterized by θ/2θ X-ray diffraction (XRD) using Cu Kα radiation. The X-ray absorption near-edge structure (XANES) and extend X-ray absorption fine structure (EXAFS) measurements at Fe K-edge were carried out at 4B9A beamline of X-ray diffraction station in Beijing Synchrotron Radiation Facility (BSRF). The magnetic measurements of the films were performed by a superconducting quantum interference device (SQUID) system as a function of magnetic field (0 to ±15 kOe). The film resistivity (ρ), magnetoresistance (MR) and Hall effect were measured using the van der Pauw four-probe configuration with the applied magnetic fields up to 11 kOe and temperature range of 10−300 K. 3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of (In1−xFex)2O3 films with x = 0, 0.03, 0.07, and 0.09. One can see that all the diffraction

Figure 1. XRD patterns of (In1−xFex)2O3 films with x = 0, 0.03, 0.07, and 0.09. The inset shows the enlarged view of (222) diffraction peaks.

peaks correspond to the cubic bixbyite structure as pure In2O3 (space group: Ia3̅). No impurity peaks are observed within the detection limit, even though the Fe concentration (x) is as high as 0.09. It is clear from the inset of Figure 1 that the (222) diffraction peaks move to higher angles with Fe doping, indicating the decreasing of average lattice spacing. This decrease in the lattice parameter suggests that the doped Fe ions substitute for the In sites of the In2O3 lattice due to the B


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Figure 2. (a) Fe K-edge XANES spectra of (In1−xFex)2O3 film with x = 0.09 as well as Fe, FeO, and Fe2O3 foils. (b) Fe K-edge EXAFS k3χ(k) oscillation functions for the (In1−xFex)2O3 film with x = 0.09 as well as Fe, FeO, and Fe2O3 foils. (c) Their Fourier transform curves: circle, experimental; solid lines, fitting.

Table 1. Best Fit Structural Parameters around Fe Atoms in the (In1−xFex)2O3 Film with x = 0.09a first shell (Fe−O)

a

structure model for FeIn1(In1‑xFex)2O3

N1

x = 0.09

4.5 ± 0.2

second shell (Fe−In1)

R1 (Å)

σ12 (10‑3 Å2)

N2

2.03 ± 0.01

8.2 ± 0.4

4.5 ± 0.1

third shell (Fe−In1)

R2 (Å)

σ22 (10‑3 Å2)

N3

R3 (Å)

σ32 (10‑3 Å2)

3.32 ± 0.005

5.1 ± 0.2

4.5 ± 0.1

3.78 ± 0.005

4.5 ± 0.2

N, R, and σ2 are the coordination number, bond length and Debye-Waller factor, respectively.

Figure 3. (a) Experimental XANES spectrum for the (In1−xFex)2O3 film with x = 0.09 and XANES spectra calculated for different model structures: FeIn1 + Ini, FeIn2 + Ini, FeIn2 + VO. (b) Experimental XANES spectrum for the (In1−xFex)2O3 film with x = 0.09 and XANES spectra calculated for different model structures: FeIn1, FeIn1 + VO, FeIn1 + 2VO.

XANES spectra in Figure 3b change obviously with introducing VO defects. The intensity of peak A remarkably decreases and

the intensity of peak B increases with introducing VO. Moreover, the calculated XANES spectrum based on model C


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films growth in an oxygen-deficient environment. From Table 2, one can see that ρ increases and nc decreases monotonically with increasing x from 0 to 0.09. It is possible that the holes from p-type Fe (the substitution of Fe2+ for In3+) partly compensate the electrons from the VO, resulting in decreasing nc, and then the scattering from more doped Fe ions may also lead to increase ρ. Figure 5a shows the temperature dependence of resistivity ρ(T) for the (In1−xFex)2O3 films with x = 0, 0.03, 0.07, and 0.09. It can be seen that the (In1−xFex)2O3 films with x = 0 and 0.03 exhibit a transition from metal to semiconductor behavior. By contrast, the (In1−xFex)2O3 films with x = 0.07 and 0.09 display a typical semiconductor behavior in the whole temperature range. Generally, the transport mechanism of semiconductor can be described as the Mott and Efros variable range hopping (VRH) at low temperature as well as hard band gap hopping at high temperature. The best understanding of transport mechanisms for the (In1−xFex)2O3 films can be achieved by the combination of the Mott VRH model, ρ ∼ exp[(T0/T)1/4], which describes those carriers hopping within localized states, as well as the hard band gap hopping model, ρ ∼ exp(Ed/KBT), which describes those carriers that have been thermally excited from localized states to the conduction band. The total resistivity of films can be expressed as ρ = A exp[(T0/T)1/4]+B exp(Ed/ KBT), where A and B are constants, T0 is the characteristic hopping temperature associated with localized states,24 and Ed is the activation energy. Figure 5b shows that, despite the varying carrier concentration nc, all lnρ versus T−1/4 curves can be well described by the combination of Mott VRH and hard band gap hopping models. The electric parameter kFl of the films can be estimated using the formula: kFl = ℏ(3π2)2/3/ (e2ρn1/3), where kF is the Fermi wave vector, l is the mean free path, ℏ is the Planck constant, e is the electron charge, ρ is the resistivity, and n is the electron concentration.25 From Table 2, the kFl > 1 for all the (In1−xFex)2O3 films, indicating that the carriers in the (In1−xFex)2O3 films are weak localized. In order to further investigate the transport mechanism of the films, we calculate the localization radius of Mott VRH, which is expressed as ζ = (2β/Nd)1/3(Tv/T0)1/4, where β is a constant, Nd is the donor concentration, and Tv is the onset temperature of Mott VRH.26 It is obvious from Table 2 that the localization radius ξ and the characteristic hopping temperature T0 show a strong dependence of Fe concentration, namely ξ decreases and T0 increases monotonically with Fe doping, suggesting that the Fe doping in the films can to some extend avert the localization effect of carriers. Investigating magnetoresistance (MR) is important for understanding the magnetic mechanism of the observed room temperature ferromagnetism and for determining the contribution of magnetic exchange interaction in oxide-based DMSs. The MR curves were measured with the applied magnetic field perpendicular to the films, defined as MR= ρH/

M4 (FeIn1 + 2VO) can well reproduce the experimental spectral features. All these results further suggest the incorporation of Fe atoms into the In1 sites with two VO in the nearest coordination shell. This also gives a strong evidence to the existence of VO around the doped Fe ions in the (In1−xFex)2O3 films. Figure 4 shows the M−H curves for the (In1−xFex)2O3 films with x = 0.03, 0.07, and 0.09, which were measured at 300 K

Figure 4. Magnetization hysteresis (M−H) curves of (In1−xFex)2O3 films with x = 0.03, 0.07, and 0.09 at 300 K. The inset shows the temperature dependence of zero field cooled (ZFC) and field cooled (FC) curves under a magnetic field of 500 Oe for the (In1−xFex)2O3 film with x = 0.09.

with the applied magnetic field parallel to the film plane. It is obvious that all the films clearly reveal hysteric behavior at 300 K, suggesting the existence of room temperature ferromagnetic behavior. The MS increases monotonously from 0.7 to 2.58 emu/cm3 when x goes from 0.03 to 0.09. This is not consistent with the previously reported cases of Fe-doped In2O3,23 where the MS decreases monotonously with the increase of Fe concentration and is attributed to an increase in the antiferromagnetic coupling between Fe ions. This needs to be further clarified, as discussed later. In order to further investigate the ferromagnetic behavior of films, the zero field cooled (ZFC) and field cooled (FC) curves for the (In1−xFex)2O3 film with x = 0.09 are shown in the inset of Figure 4. It is clear that there exists an obvious separation between the ZFC and FC curves, and the transition from ferromagnetism to paramagnetism in the temperature range of 5K to 300 K is also not observed, indicating that the film exhibits robust room temperature ferromagnetism and high Curie temperature (above 300 K). The carrier concentration nc and resistivity ρ are measured to further gain insight into the ferromagnetic mechanism for the (In1−xFex)2O3 films. The results of Hall measurements confirm n type of carriers for all the films, which is consistent with the

Table 2. Parameters of the (In1−xFex)2O3 Films with x = 0, 0.03, 0.07, and 0.09 Obtained from Hall Effect and ρ−T Measurements [Electron Concentration (n), kFl, and Mean Free Path (l)), Characteristic Hopping Temperature (T0), and Localization Radius (ξ)] (In1‑xFex)2O3 x x x x

= = = =

0 0.03 0.07 0.09

ρ (Ω·cm) 1.05 1.25 1.73 2.5

× × × ×

−3

10 10−3 10−3 10−3

n (cm−3)

kFl

l (Ǻ )

T0 (K)

ξ (nm)

× × × ×

8.21 6.59 5.01 3.58

5.51 4.43 3.56 2.61

3164 7512 26426 44917

1.741 1.583 1.217 1.099

3.6 1.1 9.4 8.6

20

10 1020 1019 1019 D


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Figure 5. (a) Resistivity-temperature (ρ−T) curves for (In1−xFex)2O3 films with x = 0, 0.03, 0.07, and 0.09. (b) Plots of lnρ versus T−1/4 for the (In1−xFex)2O3 films with x = 0, 0.03, 0.07, and 0.09.

Figure 6. Field dependence of magnetoresistance for the (In1−xFex)2O3 films with x = 0.03 (a), 0.07 (b), and 0.09 (c) measured at 10, 15, 20, and 30 K respectively. Solid lines are the theoretical fitting results. (d) Fitting parameter b as versus the inverse temperature.

ρ0. Parts a−c of Figure 6 show the MR curves for the (In1−xFex)2O3 films with x = 0.03, 0.07, and 0.09 measured at 10, 15, 20, and 30 K. One can see that the (In0.97Fe0.03)2O3 film with high carrier concentration shows obvious negative MR behavior at different temperatures. The high carrier concentration causes the film to have weak localization effect with metallic-semiconducting transition behavior. The field suppression of the weak localization may cause the negative MR.27 With further decreasing the carrier concentration, a positive contribution to MR at high field was observed at 10 K for the (In1−xFex)2O3 films with x = 0.07 and 0.09. As temperature increased to above 15K, the negative MR again dominates for

the two samples. The positive MR in DMSs may result from the spin-split of the conduction band caused by strong s-d exchange interactions.25,28 However, the positive MR can quench as the carrier concentration is high (above 1020/cm3) due to the larger Fermi energy than the spin-splitting energy,29 which is also consistent with our MR results for the (In0.97Fe0.03)2O3 film with high carrier concentration. It is obvious from Figure 6 that the Fe doping has profound effects on the positive and negative contributions to MR. With increasing Fe concentration, the positive MR contribution becomes much stronger, reflecting a stronger s-d exchange interaction. Furthermore, there exists obvious temperature E


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increases with the increase of Fe concentration, also reflecting to stronger exchange interaction between the doped Fe ions with the conduction bands of the In2O3. The detailed structural analysis using XRD, XANES, EXAFS and ab initio theoretical calculations suggest that the doped Fe ions are substitutional for the In sites and form FeIn + 2VO complex with the oxygen vacancies around Fe dopant. Therefore, the observed room temperature ferromagnetism should be due to intrinsic magnetic interaction in the (In1−xFex)2O3 films. If the ferromagnetism originates from the carrier-mediated exchange interaction, it can expect that the MS also decreases monotonically with Fe doping. However, by comparing the M−H curves as shown in Figure 4, the MS increases monotonously with Fe doping. So the ferromagnetism order in the (In1−xFex)2O3 films is not directly induced by charge carriers. Recently, a number of studies suggested that the magnetic behavior is often paramagnetic in the DMS films with high structural perfection.34,35 Kaspar et al. also reported that room temperature ferromagnetism can not be induced by additional adding n-type carriers in oxide-based DMSs and proposed that structural defect is important to mediate the magnetic interaction between TM dopants and the itinerant electrons.36 Theoretical calculations by Weng et al. found that the introducing of oxygen vacancy in DMSs can largely influence on the electronic structure of host oxides and remarkably enhance the ferromagnetism than the TM-doping only.37 Therefore, combination with these observed characteristics in XANES, Hall effect and ρ−T measurements, such as, the existence of oxygen vacancies around the Fe dopant and the Mott VRH transport behavior et al., strongly suggest that bound magnetic polarons (BMPs) interactions appear to play an important role on the ferromagnetism coupling of the (In1−xFex)2O3 films. Within BMPs, the orbitals of electrons locally trapped by oxygen vacancy overlap with the d shells of neighboring Fe, and the formation of Fe−VO−Fe groups is the key in achieving ferromagnetic ordering. One can see from Table 2 that the localization radius ξ of variable range hopping of carriers decreases with Fe doping. The reduced ξ can to some extent change the disturbance of BMPs from the hopping of carrier and shrink the radius of magnetic polaron rBMP. This can lead that more doped Fe ions are outside the rBMP and weaken the F-center mediated ferromagnetism coupling, which contracts the relationship between MS and Fe concentration in our study. According to the EDS results, the nonstoichiometric formula for the (In1−xFex)2O3 films with x = 0.03, 0.07, and 0.09 were determined to be (In 0 . 9 7 Fe 0 . 0 3 ) 2 O 2 . 3 9 , (In0.93Fe0.07)2O2.40, and (In0.91Fe0.09)2O2.42, respectively. Considering the measurement error, it can be concluded that the oxygen vacancy concentration does not almost vary with the doped Fe concentration. So the increase in MS with Fe doping is not due to the variation of oxygen vacancy concentration. Gehring and Xu et al. reported that a high value of T0 corresponds to a low probability for hopping of carriers and fewer disturbances to the exchange field of BMPs.38 So, as shown in Table 2, the higher T0 with Fe doping should lead to suppress the hopping probability of carrier and improve the stabilization of BMPs, which augmenting ferromagnetism rather weaken it. Actually, we considered that, according to the ρ−T and MR results, although the reduction in localization radius ξ with Fe doping, the enhancing T0 and s−d exchange interaction will have more positive influence on ferromagnetism, and should be the reason that increasing the M S in our (In1−xFex)2O3 films. These results indicate that the variation

dependent transition from the positive MR to the negative MR in the (In1−xFex)2O3 films with x = 0.07 and 0.09 above 15 K. This can be explained as the result of decreasing spin-splitting between the spin-polarized subbands with increasing temperature, which makes the negative MR become more pronounced at higher remperature. Moreover, the coexistence of spinrelated positive and negative MR usually occurs at the localization boundary at low temperature with semiconductor behavior,27 also implying intrinsic ferromagnetism in our (In1−xFex)2O3 films. A semiempirical expression proposed by Khosla and Fischer, including the negative and positive MR contributions was used to fit the experimental MR curves:30,31 ρH /ρ0 = 1 − a 2 ln(1 + b2H2) +

c 2H2 1 + d 2H2

(1)

a 2 = A1JD(εF )[S(S + 1) + M2 ]

(2)

⎡ ⎛ ⎞4 ⎤ g 2μB2 2 2 2JD(εF ) ⎥ ⎢ b = 1 + 4S π ⎜ ⎟ ⎢⎣ ⎝ g ⎠ ⎥⎦ (αkBT )2

(3)

2

The negative component was considered using third-order expansion of the s−d exchange Hamiltonian and the positive one was used a two-band model with different conductivities in doped In2O3. Here, J is the exchange interaction integral, S is the spin of the localized magnetic moments, D(εF) represents the density of states at the Fermi level, ⟨M⟩ represents the average magnetization. The parameters c and d are function of the mobility of carriers and conductivity in two-band, namely, c,d = F(σ1,σ2,μ1,μ2). It can be clearly seen from Figure 6a−c that the agreement between the theoretical fitting and the experimental is excellent for all the films and temperatures. Table 3 lists the values of the least-squares fitting parameters a, Table 3. MR Fitting Parameters for the (In1−xFex)2O3 films With x = 0.03, 0.07, and 0.09 at 10, 15, 20, and 30 K x 0.03

0.07

0.09

T (K)

a

b

c

d

10 15 20 30 10 15 20 30 10 15 20 30

0.403 06 0.458 94 0.480 26 0.546 85 0.322 82 0.271 35 0.225 32 0.224 72 0.252 51 0.221 04 0.207 68 0.263 23

0.818 52 0.721 76 0.664 82 0.607 87 0.8994 0.748 61 0.690 74 0.632 41 1.019 45 0.815 74 0.717 13 0.636 63

0.323 06 0.325 64 0.314 52 0.328 77 0.285 67 0.196 95 0.149 13 0.1373 0.253 73 0.173 75 0.142 23 0.163 37

0.521 82 0.466 99 0.434 12 0.402 45 0.560 35 0.468 06 0.434 21 0.413 62 0.615 06 0.494 38 0.444 24 0.419 34

b, c, and d for all the films under investigation. They exhibit a very systematic temperature and Fe concentration dependence. The fitting parameter b decreases with increasing temperature, which is observed for all the films under investigation. According to eq 3, the fitting parameter b should be inversely proportional to the temperature, which has been regarded as an evidence for the MR mechanism.32,33 Obviously the decrease of the b parameter is linear with T−1, as shown in Figure 6d. While the parameter a shows a temperature independent behavior as expected from the model. The negative MR contribution arises from spin scattering, so the slope of the b(T−1) relation F


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(6) Li, Q. H.; Wei, L.; Xie, Y. R.; Jiang, F.; Zhou, T.; Hu, G. X.; Jiao, J.; Chen, Y. X.; Liu, G. L.; Yan, S. S.; et al. Effects of Oxygen Vacancy on the Electrical and Magnetic Properties of Anatase Fe0.05Ti0.95O2−δ Films. J. Alloys Compd. 2013, 574, 67−70. (7) Mi, W. B.; Jiang, E. Y.; Bai, H. L. High-Temperature Ferromagnetism Observed in Facing-Target Reactive Sputtered MnxTi1‑xO2 Films. Acta Mater. 2008, 56, 3511−3515. (8) Sabergharesou, T.; Wang, T.; Ju, L.; Radovanovic, P. V. Electronic Structure and Magnetic Properties of Sub-3 nm Diameter Mn-Doped SnO2 Nanocrystals and Nanowires. Appl. Phys. Lett. 2013, 103, 012401. (9) Okabayashi, J.; Kono, S.; Yamada, Y.; Nomura, K. Magnetic and Electronic Properties of Fe and Ni Codoped SnO2. J. Appl. Phys. 2012, 112, 073917. (10) Zhou, T.; Wei, L.; Xie, Y. R.; Li, Q. H.; Hu, G. X.; Chen, Y. X.; Yan, S. S.; Liu, G. L.; Mei, L. M.; Jiao, J. Effects of Sn Doping on the Morphology and Properties of Fe-Doped In2O3 Eepitaxial Films. Nanoscale. Res. Lett. 2012, 7, 661. (11) Xu, T. S.; Qiao, R. M.; Tian, Y. F.; Yan, S. S.; Zhang, K.; Cao, Y. L.; Kang, S. S.; Chen, Y. X.; Liu, G. L.; Mei, L. M. Magnetoresistance and Electron-Hole Exchange Interaction in (Co1‑xInx)2O3‑v Concentrated Ferromagnetic Semiconductors. Appl. Phys. Lett. 2013, 103, 202411. (12) Sun, Q. B.; Zeng, Y. P.; Jiang, D. L. Controlling Magnetic Properties of Fe3+ Doped Indium Oxide Nanocubes by Atmospheric Annealing Method. J. Appl. Phys. 2011, 110, 083922. (13) Jiang, F. X.; Xi, S. B.; Ma, R. R.; Qin, X. F.; Fan, X. C.; Zhang, M. G.; Zhou, J. Q.; Xu, X. H. Room-Temperature Ferromagnetism in Fe/Sn-Codoped In2O3 Powders and Thin Films. Chin. Phys. Lett. 2013, 30, 047501. (14) Peleckis, G.; Wang, X. L.; Dou, S. X. Room-Temperature Ferromagnetism in Mn and Fe Codoped In2O3. Appl. Phys. Lett. 2006, 88, 132507. (15) Yoo, Y. K.; Xue, O. Z.; Lee, H. C.; Cheng, S.; Xiang, X. D.; Dionne, G. F.; Xu, S.; He, J.; Chu, Y. S.; Preite, S. D.; et al. Bulk Synthesis and High-Temperature Ferromagnetism of (In1−xFex)2O3−σ with Cu Co-Doping. Appl. Phys. Lett. 2005, 86, 042506. (16) Chu, D. W.; Zeng, Y. P.; Jiang, D. L. Abnormal Phase Transition and Magnetic Properties in Cu, Fe Co-Doped In2O3 Nanocrystals. Appl. Phys. Lett. 2008, 92, 182507. (17) An, Y. K.; Wang, S. Q.; Feng, D. Q.; Wu, Z. H.; Liu, J. W. Correlation Between Oxygen Vacancies and Magnetism in Fe-Doped In2O3 Films. Appl. Surf. Sci. 2010, 276, 535−538. (18) Sun, S. H.; Wu, P.; Xing, P. F. d0 Ferromagnetism in Undoped n and p-Type In2O3 Films. Appl. Phys. Lett. 2012, 101, 132417. (19) Yan, S. M.; Qiao, W.; Zhong, W.; Au, C. T.; Dou, Y. W. Effects of Sites Occupancy and Valence State of Fe Ions on Ferromagnetism in Fe-doped In2O3 Diluted Magnetic Semiconductor. Appl. Phys. Lett. 2014, 104, 062404. (20) Xing, P. F.; Chen, Y. X.; Yan, S. S.; Liu, G. L.; Mei, L. M.; Zhang, Z. Tunable Ferromagnetism by Oxygen Vacancies in Fe-Doped In2O3 Magnetic Semiconductor. J. Appl. Phys. 2009, 106, 043909. (21) Li, S. C.; Ren, P.; Zhao, B. C.; Xia, B.; Wang, L. Room T e m p e r a t u r e Fe r r o m a g n e t i s m of B u l k P o l y c r y s t a l l i n e (In0.85‑xSnxFe0.15)2O3: Charge Carrier Mediated or Oxygen Vacancy Mediated? Appl. Phys. Lett. 2009, 95, 102101. (22) Panguluri; Raghava, P.; Kharel, P.; Sudakar, C.; Naik, R.; Suryanarayanan, R.; Naik, V. M.; Petukhov, A. G.; Nadgorny, B.; Lawes, G. Ferromagnetism and Spin-Polarized Charge Carriers in In2O3 Thin Flms. Phys. Rev. B 2009, 79, 165208. (23) Xu, X. H.; Jiang, F. X.; Zhang, J. X.; Fan, C.; Wu, H. S.; Gehring, G. A. Magnetic and Transport Properties of n-Type Fe-Doped In2O3 Ferromagnetic Thin Films. Appl. Phys. Lett. 2009, 94, 212510. (24) Mott, N. F.; Davies, E. A. Electronic Processes in Noncrystalline Materials, 2nd ed.; Oxford University: New York, 1979. (25) Xu, Q. Y.; Hartmann, L.; Schmidt, H.; Hochmuth, H.; Lorenz, M.; Spemann, D.; Grundmann, M. s-d Exchange Interaction Induced Magnetoresistance in Magnetic ZnO. Phys. Rev. B 2007, 76, 134417.

of localization effect could strongly modify the ferromagnetism in the films with BMPs scenario and also point toward a strong relationship between the oxygen vacancies and the ferromagnetic interactions. However, further experimental and theoretical investigations are needed to address this issue.

4. CONCLUSIONS In summary, the systematic studies on the local Fe structure, magnetic and transport properties of the (In1−xFex)2O3 films grown by RF-magnetron sputtering technique were performed by XRD, XAFS, Hall effect, ρ−T, MR. and magnetic measurements. The Fe K-edge XANES spectra combined with multiple-scattering ab initio calculations reveal that the doped Fe ions are incorporated into the In sites and form FeIn1+2 VO complex with oxygen vacancies around Fe dopant. The typical room temperature ferromagnetism is observed for all the (In1−xFex)2O3 films. The MS increases monotonically with the increase of Fe concentration, while carrier concentration nc decreases monotonically with Fe doping, suggest that the ferromagnetism is not carrier mediated. The Mott VRH and Hard band gap hopping transport behavior dominates the conduction mechanism of the films, confirming that the carriers are localized. The (In1−xFex)2O3 films with low Fe concentration show negative MR. A increase in Fe concentration results in increasing the positive MR contribution, reflecting the occurrence of spin polarization and stronger s−d exchange interaction. The observed room temperature ferromagnetism in the (In1−xFex)2O3 films can be explained using bound magnetic polarons associated with oxygen vacancies and the change of localization effect can remarkably influence the ferromagnetic order of the films.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-022-60214028. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No 10904110, 11174217) and by the Beijing Synchrotron Radiation Laboratory.

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REFERENCES

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Oxygen Vacancy-Induced Room Temperature Ferromagnetism and

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